This invention relates to the manufacture of thin-film field effect transistors, and in particular to a method of treating the channel region of the transistor so as to reduce off-current leakage.
In solid state electronic devices thin film field effect transistors (TFTs) are commonly used as switches. In particular, in imaging and display devices, a TFT is commonly associated with each pixel to enable a respective electrical signal to be coupled to or from each individual pixel. One of the important performance factors of the TFT is low off-current, or leakage; low leakage is desirable so that the display or imager exhibits low noise, and so that an imager has a linear response to incident light. Off-current refers to the leakage current between the source and drain electrodes with the gate electrode voltage a few volts or more below the threshold voltage, that is the voltage at which the transistor starts to conduct significantly.
Imager and display arrays are typically fabricated on large substrates on which many components, including TFTs, address lines, and devices such as photosensors, are formed through the deposition and patterning of layers of conductive, semiconductive, and insulative materials. The TFT fabrication process involves several patterning steps to produce the desired arrangement of a channel region between a source and a drain electrode with the desired arrangement of semiconductive material disposed between the electrodes and over the gate electrode. The condition of the channel region of a TFT after this fabrication process can effect the off-current of the device. Common passivation techniques, such as the deposition of a dielectric layer over the channel region of the TFT after it has been fabricated, have not provided reproducibly low off-current leakage results. Another approach to passivation is disclosed in U.S. Pat. No. 4,704,783 of Possin et al. (Possin '783), which patent is assigned to the assignee of the present invention and incorporated herein by reference. The passivation process of Possin '783 includes removing a portion of the semiconductor material at the bottom of the channel region gap, plasma etching the region, exposing the region to monoethanolamine solution, rinsing with distilled water, and air baking prior to depositing a dielectric material passivation coating. As reported in Possin '783, this passivation method advantageously reduced off-current leakage in TFTs to a nominal value of about 1 picoampere (as compared with about 50 picoamperes in a TFT in which the passivating dielectric was deposited without the preliminary treatment).
It is thus an object of this invention to provide a TFT fabrication method for reliably passivating the channel region of a TFT to provide low off-current TFTs.